In conventional radiation imaging arrangements, collimators are used to permit only beams of radiation emanating along a particular path to pass a selected point or plane. Collimators are frequently used in radiation imagers to ensure that only radiation beams passing along a direct path from the known radiation source strike the detector thereby minimizing detection of beams of scattered or secondary radiation.
Particularly in radiation imagers used for medical diagnostic analysis or for non-destructive evaluation procedures, it is important that only radiation emanating from a known source and passing along a direct path from that source be detected and processed by the imaging equipment. If the detector is struck by undesired radiation such as that passing along non-direct paths to the detector, performance of the imaging system can be compromised.
One diagnostic technology that incorporates collimators is the gamma camera typically utilized in Single Photon Emission Computed Tomography (SPECT) scanning, which is a nuclear medicine procedure in which gamma camera(s) have traditionally rotated around the patient taking pictures from multiple angles. From these images, a computer is employed to form a tomographic image of the internal area-of-interest within the patient using a calculation process that is similar to that used in X-ray Computed Tomography (CT) and in Positron Emission computed Tomography (PET).
In the instance of SPECT scanning, a subject (patient) is infused with a radioactive substance that emits gamma rays. Conventionally, a gamma camera includes a transducer to receive the gamma rays and record an image therefrom. In order for the image to be a true representation of the subject being investigated, a collimator having collimating apertures is positioned between the transducer and the subject to screen out all of the gamma rays except those directed along a straight line through the collimating apertures between a particular part of the subject and a corresponding particular part of the transducer. Traditionally, the collimator is made of a radiopaque material such as lead and has collimating apertures, which have been formed therein by various means such as drilling, casting, or lamination of corrugated strips of lead foil.
With current systems, the number of angular views that can be acquired of a target organ, (e.g. the heart), by a SPECT gamma camera is restricted to one-view per gantry stop per detector. Acquisition of multiple views requires rotation (or at least movement) of the camera. Hence, tomographic imaging of rapid tracer dynamics (uptake and washout from various tissue and metabolic compartments) is difficult due to the necessity of scanning large massive detectors rapidly.
Bisegmental collimators, such as those described in U.S. Pat. No. 4,659,935 to Hawman, are known for improving the sensitivity of SPECT in imaging small organs. Recently, multisegmental diverging collimation has been proposed as method that can achieve higher sensitivity than multipinhole SPECT systems. It has also been realized that multisegment parallel collimation, using parallel-hole segments, but having different slant angles, can provide even more sensitivity for the same system spatial resolution.
Fabrication of a multi-view composite collimator entails the abutment of multiple collimator segments having large differences in the direction of view. Since multichannel collimators are typically formed from lead, which has a finite attenuation coefficient for gamma rays, the collimator is of finite thickness, typically a few (e.g., 2 to 4) centimeters. Regions between collimator segments in such composite collimators are often filled with a radiopaque material, such as lead. This results in gaps (i.e., uncollimated space) on the detector, which have a width approximately equal to the collimator thickness times the sum of the tangents for the slant angle of the abutting slanted collimator segments.
It would be desirable to minimize such gaps or dead zones in multi-view composite collimators in order to maximize the amount of collimated area of the detector. Such an arrangement should improve the sensitivity and imaging speed of a SPECT systems.